1. Field of the Invention
The present invention is generally directed toward a method and system for inhibiting torque steer in a vehicle equipped with steerable wheels that are power driven.
2. Description of Related Art
Vehicles equipped with steerable wheels that are power driven such as front-wheel drive vehicles and four-wheel drive vehicles have the potential to generate a difference in left/right side tire longitudinal force under the application of engine torque. This difference in left/right side tire longitudinal force can be observed in most vehicles, but is especially noticeable in vehicles equipped with a traction enhancement device such as a limited slip differential or another type of torque splitting control device. The mismatch in left/right driving torque creates a difference in the suspension restoring torque between the left side and the right side of the vehicle that ultimately leads to perturbations in steering wheel torque, which is commonly referred to as “torque steer”.
The dynamic conditions that operate to cause torque steer in a vehicle equipped with power-driven, steerable wheels, are well known in the art. Generally, when the vehicle's power-driven steerable wheels are turned to the left under the application of engine torque, the left side tire longitudinal force is smaller than the right side tire longitudinal force. This translates into a torque steer that the driver of the vehicle feels in the steering wheel as a pull to the left. Factors such as the amount of engine torque applied and the transmission gear selected contribute to the overall level of driving torque delivered to the front axle, the resulting left/right driving torque difference amount, and the resulting level of torque steer.
A variety of traction control systems are known that control the slip rate of the driving axle in order to enhance vehicle stability and maneuverability. These known traction control systems generally become active upon the occurrence of a wheel-slip condition or upon the occurrence of a difference in driving wheel speed. Upon sensing such a condition, such systems may incorporate engine throttle or torque control and/or brake system control to improve traction and to mitigate torque steer. The intent of such systems is to intervene in the event of excessive wheel slip so as to keep the tire slip rate within a desired range.
One such system is described in Schmitt et al., U.S. Pat. No. 6,154,546. This patent discloses a method and device for controlling traction in a motor vehicle in which a maximum transmittable driving torque is calculated as a function of various operating parameters of the vehicle and its turning performance. When a skidding tendency of at least one driving wheel occurs, the system engages and reduces engine torque to a calculated maximum transmittable torque value.
In many driving situations however, the left/right difference in longitudinal tire force can lead to a persistent torque steer before the onset of appreciable wheel slip. This situation is especially problematic when high levels of engine torque are applied as a vehicle is being steered in a direction other than straight on a high adhesion surface. In such situations, the torque steer condition occurs before a typical wheel-slip-based traction control system activates. Thus, conventional wheel-slip based traction control systems are generally ineffective to mitigate or inhibit torque steer before a wheel-slip condition occurs.